1. Field of the Invention
This invention relates to a continuous annealing apparatus having finely tiltable cylindrical rolls for guiding beltlike metals or metal strips adjacent to hearth rolls in a high temperature heat-treatment region of a vertical continuous annealing furnace for use in heat-treatment of the metal strips.
2. Description of the Prior Art
A continuous annealing furnace generally includes in series from an upstream to a downstream side a heating, a soaking and a cooling zone through which metal strips progressively pass so as to be properly heat-treated in accordance with various purposes.
In operation with such an apparatus, metal strips are often meandering or staggering in traverse directions and are buckling in the strips.
In order to avoid the meandering of the metal strips, it has been proposed to use hearth rolls having larger diameters at their midportions in the same manner as in belt pulleys. FIGS. 1a and 1b illustrate examples of such hearth rolls. The hearth roll shown in FIG. 1a has tapered ends. The hearth roll shown in FIG. 1b has a crowning outer circumferential surface. These hearth rolls serve to a certain extent to prevent metal strip from meandering. When tapered angles .theta. as shown in FIG. 1a or crowning amounts or curvatures 1/.rho. shown in FIG. 1b are too large or tensile forces in longitudinal directions in the metal strips increase, then compressive stresses are caused by uneven tensile forces in the metal strips, which in turn give rise to buckling of the metal strips.
When the tapered hearth rolls 1a are used, the metal strips are likely to cause bucklings 3 at locations corresponding to taper starting lines of the rolls. Such bucklings spoil the appearance of the metal strip so as to lose their worth as products. In some extreme cases, moreover, the metal strips are broken off due to the bucklings resulting in a great trouble.
On the other hand, when the tapered angles or crowning amounts are reduced in order to avoid the bucklings, the performance of the hearth rolls preventing the meandering of the metal strips is lost to make impossible the proper transferring of the metal strips. In extreme cases, the metal strips are detached from the rolls and edges of the strips scrape the furnace walls to cause large troubles.
FIG. 3 is a graph symbolically illustrating this fact. A lower left shaded portion is a meandering zone and an upper right shaded portion is a buckling zone. Between these zones there is a narrow zone in which any meandering and buckling of metal strips do not occur. It is clearly evident that there are limited proper values of the tapered angles .theta. and curvatures 1/.rho. of the hearth rolls. In general, metal strips to be treated in the continuous annealing furnaces are in wide ranges of dimension (thickness and width) and material (strength at high temperature and heat-treatment condition). The proper zone shown in FIG. 3 varies with these factors of metal strips. However, the shape of the hearth rolls were determined when the plant was constructed. Accordingly, the adaptability to the variation in metal strips is insufficient to avoid the buckling and meandering of the metal strips.
The inventors have proposed to use auxiliary small diameter rolls in order to prevent the buckling of metal strips due to the hearth rolls of annealing furances (Japanese Patent Application No. 188,257/82). The method using the auxiliary small diameter rolls exhibits significant effect for preventing the buckling but does not exhibit any improved effect for controlling the meandering in comparison with the prior art.